So I was looking back at the original schematic that I'd reverse engineered and got to wondering about the path for the clock signal.

While several ICs (CPU, 6840 timer IC, 6821 PIA, and the lower half of RAM) are driven directly from the 1MHz oscillator, other parts of the clock signal path are combined with other signals:

The chip select for the ROM is generated by a NAND of the A15 address line and the clock.

The chip select for the 6840 and 6821 ICs is generated by NAND of the clock and A15 (inverted).

The clock bus that continues to the interface board is fed through a NAND gate with the reset line.

On the interface board, this modified clock goes directly to the upper half of RAM.

The clock for the 8279 display / keyboard driver is fed through a 74LS375 latch.

The circuit that splits the R/W line into separate /RD and /WR lines for the 8279 is also OR'd with the clock.

Is there a reason for all this, or did someone overthink the design? (see attachment for a simplified schematic of my post)

 

The reason is centered around the presence of peripheral chips using the Motorola control signal scheme and the Intel control signal scheme. It is an attempt to create both sets of control signals so that chips from different families can be used. Not sure why you might think somebody was over thinking.

 

I hope that you are well aware of the two design schemes.

Motorola scheme is a common E clock and one R/W.
Intel scheme is /RD and /WR.

 

So I should be able to take all the clock gating like so?

 

Epilogue: failure.

While trying to figure out why the display board I built refuses to light up at all, I accidentally shorted the 5V and 12V supplies together. Smoke came out of the 74LS04 on the logic board, a 'single gate' IC was rendered a dead short. While the other, more important, ICs (HC11, PIA, PTM, display controller) do not appear to be shorted, it is necessary to assume they have all been damaged.

>.<

I still plan to try and see if I can figure out the interrupt code, but the rest...

 

Epilogue: failure.

While trying to figure out why the display board I built refuses to light up at all, I accidentally shorted the 5V and 12V supplies together. Smoke came out of the 74LS04 on the logic board, a 'single gate' IC was rendered a dead short. While the other, more important, ICs (HC11, PIA, PTM, display controller) do not appear to be shorted, it is necessary to assume they have all been damaged.

>.<

I still plan to try and see if I can figure out the interrupt code, but the rest...

Chips are powered by Smoke
when the smoke escapes , the chip fails
Other chips in the area smell this smoke
and are upset,
they either stop working or work intermittently in mourning !!!